In optical fiber communication systems the quality of an optical signal gradually decreases as it propagates along the fiber. The following various effects impair the signal.                Accumulation of amplified spontaneous emission (ASE) noise originating from optical amplifiers, i.e. gradual decrease of the optical signal-to-noise ratio (OSNR)        Chromatic dispersion        Nonlinear effects (self phase modulation, cross phase modulation, four wave mixing, etc.)        Polarization mode dispersion        
The accumulation of ASE noise does not change the optical signal waveform. However, at the receiver, where the optical signal is converted into an electrical current, the beat noise between the data signal and the ASE light, and the beat noise of the ASE light with itself cause electrical noise.
The latter three effects distort the optical signal waveform. The distortions due to chromatic dispersion can in principle be compensated by a dispersion compensating fiber. However, the interplay of chromatic dispersion with self- and cross-phase modulation as well as other nonlinear effects and polarization mode dispersion result in irrecoverable impairment.
In the evaluation of an optical fiber communication system, the bit error rate (BER) is usually used as a parameter for the system performance. The BER is defined as the probability that a bit is erroneously received, i.e. the probability that either a logical one (mark) is detected as a zero or a logical zero (space) as a one. The BER is given by
            BER      ⁡              (        x        )              =                  1        2            ⁡              [                                            ∫                              -                ∞                            x                        ⁢                                                            pdf                  1                                ⁡                                  (                                      x                    ′                                    )                                            ⁢                              ⅆ                                  x                  ′                                                              +                                    ∫              x              ∞                        ⁢                                                            pdf                  0                                ⁡                                  (                                      x                    ′                                    )                                            ⁢                              ⅆ                                  x                  ′                                                                    ]              ,where pdf1 and pdf0 are the probability distribution functions of the detected voltages of marks and spaces, respectively at the center of the eye pattern, and x is the decision threshold.
Provided that the ASE noise causes the most severe impairment, the noise at the receiver in conventional systems without all-optical regeneration can be modeled as Gaussian noise (see non-patent document 1, for example). The probability distribution functions of the detected voltages of marks and spaces can thus be approximated by
                    pdf        i            ⁡              (        x        )              =                  1                              2                    ⁢                      σ            i                              ⁢              exp        (                                            (                              x                -                                  μ                  i                                            )                        2                                2            ⁢                          σ              i              2                                      )              ,          ⁢      (                  ⅈ        =        0            ,      1        )  where μ1 and μ0 are the mean values and σ1 and σ0 the standard deviations for marks and spaces, respectively.
In the Gaussian approximation, the BER at the optimized threshold voltage can be expressed analytically as
  BER  =            1      2        ⁢          erfc      (              Q                  2                    )      with the Gaussian error function
      erfc    ⁡          (      y      )        =            ∫      y      ∞        ⁢                  ⅇ                              -                          ξ              2                                /          2                    ⁢              ⅆ        ξ            and the so-called Q-factor (see non-patent document 2, for example)
  Q  =                              μ          1                -                  μ          0                                      σ          0                +                  σ          1                      .  
In order to regain the OSNR and eliminate signal distortions regenerators are used. Conventional regenerators consist of an optical receiver and a transmitter. The receiver converts the optical signal to an electrical signal. The receiver comprises an electrical clock extraction circuit and a decision circuit, which discriminates the incoming marks and spaces. The transmitter converts the thus regenerated electrical signal back into an optical signal.
In the case of all-optical regenerators the conversion into an electrical signal is avoided by processing the signal utilizing nonlinear optical effects. While 2R regenerators only re-amplify and re-shape (therefore 2R) the optical signal, 3R regenerators also perform re-timing (therefore 3R).
The central component of an all-optical regenerator is a nonlinear device that provides a nonlinear transfer function between optical input and output power. A variety of all-optical nonlinear devices have been demonstrated, such as                semiconductor optical amplifiers (SOAs)        nonlinear optical loop mirrors (NOLMs)        saturable absorber switches (see patent document 1 and non-patent document 3, for example)        four wave mixing (see non-patent document 4, for example)        polarization rotation switch (see non-patent document 5, for example)        spectrally broadening fiber (see non-patent document 6 and patent document 2, for example).        
The function of an ideal nonlinear device with a typical transfer function as depicted in FIG. 1 is to redistribute the pdfs of marks and spaces. In the ideal case, this transfer function is bit pattern independent. If an ideal receiver is used to detect the signal before and after the nonlinear device at an optimized decision threshold, the BER stays the same. If, however, further ASE noise is added behind the nonlinear device, the BER does not stay the same and depends strongly on the shape of the output pdf after the nonlinear device. Accordingly, the BER can be minimized if the pdf is redistributed in an optimized way.
In systems with all-optical regenerators, the Gaussian approximation of the pdfs and thus the concept of the Q-value can be used only as a rough approximation due to the nonlinear transformation of the pdfs of marks and spaces. The signal quality needs to be quantified by direct BER measurements at an optimized decision threshold or by measuring the combined pdf of marks and spaces in the center of the eye pattern. There is a method of monitoring the quality of the optical signal at the site of 2R or 1R (only re-amplify) regenerator without performing a clock recovery operation (see patent document 3, for example). The optimization of the decision threshold when measuring the BER requires sweeping the decision threshold voltage.
Optical regenerators not only require means to monitor the optical signal but also means to transmit the monitor information over the transmission system. A monitoring system for all-optical regenerators and an all-optical regenerator employing this monitoring system have already been disclosed (see patent documents 4 and 5, for example).
Non-Patent Document 1
Ed. Kaminov and Koch, “Optical Fiber Telecommunications IIIA”, Chapter 10, Academic Press, pp. 302-335, 1997
Non-Patent Document 2
N. S. Bergano, F. W. Kerfoot, and C. R. Davidson, “Margin measurements in optical amplifier systems”, IEEE Photonics Technology Letters, Vol. 5, No. 3, pp. 304-306, March 1993
Non-Patent Document 3
A. Hirano, H. Tsuda, H. Kobayashi, R. Takahashi, M. Asobe, K. Sato, and K. Hagimoto, “All-optical discrimination based on nonlinear transmittance of MQW semiconductor optical gates”, Journal of Lightwave Technology, Vol. 17, No. 5, pp. 873-884, May 1999
Non-Patent Document 4
E. Ciaramella and S. Trillo, “All-optical signal reshaping via four-wave-mixing in optical fibers”, IEEE Photonics Technology Letters, Vol. 12, No. 7, pp. 849-851, July 2000
Non-Patent Document 5
M. Zhao, J. D. Merlier, G. Morthier, and R. Baets, “All-optical 2R regeneration based on polarization rotation in a linear optical amplifier”, IEEE Photonics Technology Letters, Vol. 15, No. 2, pp. 305-307, February 2003
Non-Patent Document 6
P. V. Mamyshev, “All-optical data regeneration based on self-phase modulation effect”, ECOC'98 September 1998, Madrid, Spain, pp. 475-476
Patent Document 1
U.S. Pat. No. 6,504,637 B1
Patent Document 2
U.S. Pat. No. 6,141,129
Patent Document 3
U.S. Pat. No. 6,433,899 B1
Patent Document 4
U.S. Pat. No. 5,657,154
Patent Document 5
publication of Japan patent application, H08-288916